The black rat (Rattus rattus) is a common long-tailed rodent of the genus Rattus (rats) in the subfamily Murinae (murine rodents). The species originated in tropical Asia and spread through the Near East in Roman times before reaching Europe by the 1st century and spreading with Europeans across the world.

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Taxonomy

The black rat was one of the many species originally described by Linnaeus in his 18th century work, Systema Naturae, and it still bears its original name of Rattus rattus.[2] It is the type species of the genus Rattus. Alternate names include ship rat, roof rat, house rat, Alexandrine rat, and old English rat.

Origin of Rattus rattus

Rattus rattus bone remains that date back to the Norman Period have been discovered in Britain. Evidence also suggests that R. rattus existed in prehistoric Europe as well as Levant (eastern Europe) during post-glacial periods.[5] The specific origin of the black rat is uncertain due to the rat's disappearance and reintroduction. Evidence such as DNA and bone fragments also suggests that rats did not originally come from Europe, but migrated from southeast Asia.[6]

Rats are resilient vectors for many diseases because of their ability to hold so many infectious bacteria in their blood. Rats played a primary role in spreading bacteria, such as Yersinia pestis, which is responsible for the Justinianic plague and Bubonic plague.[6] According to epidemiological models, Yersinia pestis originated outside of Europe which indicates that Western and central Europe have never had any natural rodent plagues.
The modern Roman rat arose from an ancestor that originated in Malaysia.[6] The number of chromosomes these Malaysian rats and the Mediterranean Black Rats differ by four chromosomes.[6] Therefore, it seems that speciation could have occurred when the rats colonized southwest India, which was a primary country where Romans obtained their spices. Because Rattus rattus is a passive traveler, they could have easily traveled to Europe during the trading between Rome and the southwestern Asian countries. Evidence also suggests that, in 321-331 BC, Egyptian birds were preying on Mediterranean rats, though this is not enough to prove that Egypt was the source of the rats.[6]

Diet

Black rats are considered omnivores and eat a wide range of foods, including seeds, fruit, stems, leaves, fungi, and a variety of invertebrates and vertebrates. They are generalists, and thus not very specific in their food preferences, which is indicated by their tendency to feed on any meal provided for cows, swine, chickens, cats, and dogs.[7] They are similar to the tree squirrel in their preference of fruits and nuts. They eat about 15 grams per day and drink about 15 ml per day.[8] Their diet is high in water content.[7] They are a threat to many natural habitats because they feed on native birds and insects. They are also a threat to many farmers since they feed on a variety of agricultural-based crops, such as cereals, sugar cane, coconuts, cocoa, oranges, and coffee beans.[9]

Distribution and habitat

The black rat originated in India and Southeast Asia, and spread to the Near East and Egypt, and then throughout the Roman Empire, reaching England as early as the 1st century.[10]Europeans subsequently spread it throughout the world. The black rat is, again largely confined to warmer areas, having been supplanted by the brown rat (Rattus norvegicus) in cooler regions and urban areas. In addition to being larger and more aggressive, the change from wooden structures and thatched roofs to bricked and tiled buildings favored the burrowing brown rats over the arboreal black rats. In addition, brown rats eat a wider variety of foods, and are more resistant to weather extremes.[11]

Black rat populations can explode under certain circumstances, perhaps having to do with the timing of the fruiting of the bamboo plant, and cause devastation to the plantings of subsistence farmers; this phenomenon, Mautam, is happening currently in parts of India.[12]

Black rats are thought to have arrived in Australia with the First Fleet, and subsequently spread to many coastal regions in the country.[13]

In New Zealand, black rats have an unusual distribution and importance, in that they are utterly pervasive through native forests, scrublands, and urban parklands. This is typical only of oceanic islands that lack native mammals, especially other rodents. Throughout most of the world, black rats are found only in disturbed habitats near people, mainly near the coast. Black rats are the most frequent predator of small forest birds, invertebrates, and perhaps lizards in New Zealand forests, and are key ecosystem changers. Controlling their abundance on usefully large areas of the New Zealand mainland is a crucial current challenge for conservation managers.

Black rats adapt to a wide range of habitats. In urban areas they are found around warehouses residential, buildings, and other human settlements. They are also found in agricultural areas, such as in barns and crop fields. In urban areas they prefer to live in dry upper levels of buildings, so they are commonly found in wall cavities and false ceilings. In the wild, black rats live in cliffs, rocks, the ground, and trees.[9] They are great climbers and prefer to live in trees, such as pines and palm trees. Their nests are typically spherical and made of shredded material, including sticks, leaves, other vegetation, and cloth. In the absence of trees, they can burrow into the ground.[8] Black rats are also found around fences, ponds, riverbanks, streams, and reservoirs.[7]

The black rat, along with the brown rat, is one of the most widespread rats and animal species in the world.

Home range

Home range refers to the area in which an animal travels and spends most of its time. It is thought that male and female rats have similar sized home ranges during the winter, but male rats increase the size of their home range during the breeding season. Along with differing between rats of different gender, home range also differs depending on the type of forest in which the black rat inhabits. For example, home ranges in the beech forests of South Island, New Zealand appear to be much larger than the non-beech forests of North Island. Due to the limited number of rats that are studied in home range studies, the estimated sizes of rat home ranges in different rat demographic groups are inconclusive.

Behavior

It is nocturnal and omnivorous, with a preference for grains and fruit. Compared to the brown rat, it is a poor swimmer, but more agile and a better climber, tending even to flee upwards. In a suitable environment it will breed throughout the year, with a female producing three to six litters of up to ten young. Females may regulate their production of offspring during times when food is scarce, producing as few as only one litter a year. R. rattus lives for about 2–3 years. Social groups of up to sixty can be formed.

Nesting behavior

Through the usage of tracking devices such as radio transmitters, rats have been found to occupy dens located in trees, as well as on the ground. In Puteki, a kauri forest, rats have been found to form dens together. Rats appear to den and forage in separate areas in their home range depending on the availability of food resources.[14] Research shows that in New South Wales, the black rat prefers to inhabit lower leaf litter of forest habitat. There is also an apparent correlation between the canopy height and logs and the presence of black rats. This correlation may be a result of the distribution of the abundance of prey as well as available refuges for rats to avoid predators. As found in North Head, New South Wales, there is positive correlation between rat abundance, leaf litter cover, canopy height, and litter depth. All other habitat variables showed little to no correlation.[15]

Foraging behavior

As generalists, Black rats express great flexibility in their foraging behavior. They are predatory animals and adapt to different micro-habitats. They often meet and forage together in close proximity within and between sexes.[14] Rats tend to forage after sunset. If the food cannot be eaten quickly, they will search for a place to carry and hoard to eat at a later time.[7]
Although black rats eat a broad range of foods, they are highly selective feeders; only a restricted number of the foods they eat are dominant foods.[16] When black rat populations are presented with a wide diversity of foods, they eat only a small sample of each of the available foods. This allows them to monitor the quality of foods that are present year round, such as leaves, as well as seasonal foods, such as herbs and insects. This method of operating on a set of foraging standards ultimately determines the final composition of their meals. Also, by sampling the available food in an area, the rats maintain a dynamic food supply, balance their nutrient intake, and avoid intoxication by secondary compounds.[16]

Very few people currently keep black rats as pets. Most pet rats (or fancy rats) are domesticated brown rats.

Predators and parasites

The black rat serves as prey to cats and owls in domestic settings. In less urban settings, rats are preyed upon by weasels, foxes, and coyotes. These predators have little effect on the control of the black rat population because black rats are agile and fast climbers. In addition to agility, the black rat also makes use of its keen sense of hearing to detect danger and quickly evade mammalian and avian predators.[7]
Rats serve as outstanding vectors for transmittance of diseases because they have the ability to carry bacteria and viruses in their systems. A few parasites that are common to rats include Streptococcus pnuemoniae, Corynebacterium kutsheri,Bacillus piliformis, Pasteurella pneumotropica, and Streptobacillus moniliformis, to name a few. All of these parasites are disease causing agents in humans. In some cases, these diseases are incurable.[20]

R. rattus as an invasive species

Damage caused by R. rattus

After Rattus rattus was introduced to the northern islands of New Zealand, it has influenced them by feeding on the seedlings in the area. Even after eradication of R. rattus the negative effects may take decades to reverse. When consuming these seabirds and seabird eggs, these rats reduce the pH of the soil. This harms plant species by reducing nutrient availability in soil, thus decreasing the probability of seed germination. For example, research conducted by Hoffman et al. indicates a large impact on sixteen indigenous plant species directly preyed on by R. rattus. These plants displayed a negative correlation in germination and growth in the presence of black rats.[21]
Rats prefer to forage in forest habitats. In the Ogasawara islands, they prey on the indigenous snails and seedlings. Snails that inhabit the leaf litter of these islands showed a significant decline in population upon the introduction of Rattus rattus. The black rat shows preference for snails with larger shells (greater than 10mm). This is the reason for the great decline in population of snail with larger shells upon the introduction of black rats to the Ogasawara islands of Japan. This interaction is a result of the lack of prey refuges, which causes these snails to be less adapted to avoiding the black rat.[22]

Complex pest

The black rat has been considered a complex pest, which is a pest that influences the environment in both harmful and beneficial ways. In many cases, after the black rat is introduced into a new area, the population size of some native species declines or goes extinct altogether. This is often due to the fact that the black rat is a good generalist with a wide dietary niche and a preference for complex habitats; this causes strong competition for resources among small animals. This has led to the black rat completely displacing many native species in Madagascar, the Galapagos, and the Florida Keys. In a study by Stokes et al., habitats suitable for the native bush rat, Rattus fuscipes, of Australia are often invaded by the black rat and are eventually occupied by only the black rat. When the abundances of these two rat species were compared in different micro-habitats, both were found to be affected by micro-habitat disturbances, but the black rat was most abundant in areas of high disturbance; this indicates it has a better dispersal ability.[23]
Despite the black rats tendency to displace native species, it can also aid in increasing species population numbers and maintaining species diversity. The bush rat, a common vector for spore dispersal of mycorrhiza commonly known as truffles, has been extirpated from many micro-habitats of Australia. In the absence of a vector for spore dispersal of these truffles, the diversity of truffle species will decline. In a study conducted by Vernes et al. in New South Wales, Australia it was found that although the bush rat consumes a diversity of truffle species, the black rat consumes as much of the diverse fungi as the natives and is an effective vector for spore dispersal. Since the black rat now occupies the many of the micro-habitats that were previously inhabited by the bush rat, the black rat plays an important ecological role in the dispersal of fungal spores. By eradicating the black rat populations in Australia, the diversity of fungi would decline, potentially doing more harm than good.[23]

Control methods

Large-scale rat control programs have been taken to maintain a steady level of the invasive predators in order to conserve the native species in New Zealand such as kokako and mohua.[24] Pesticides, such as pindone and 1080 (sodium fluoroacetate), are commonly distributed via aerial spray by helicopter as a method of mass control on islands infested with invasive rat populations. Bait, such as brodifacoum, is also used along with colored dyes in order to kill and identify rats for experimental and tracking purposes. Another method to track rats is the use of wired cage traps, which are used along with bait, such as rolled oats and peanut butter, to tag and track rats to determine population sizes through methods like mark-recapture and radio-tracking.[14] Poison control methods are effective in reducing rat populations to nonthreatening sizes, but rat populations often rebound to normal size within months. Besides their highly adaptive foraging behavior and fast reproduction, the exact mechanisms for their rebound is unclear and are still being studied.[25]

Endangerment and conservation

Although predators like R. rattus are often a threat to native species on islands, there is evidence that R. rattus is becoming endangered in Great Britain. Rattus rattus populations were common in Great Britain, but began to decline after the introduction of the brown rat in the eighteenth century. R. rattus populations remained common in seaports and major cities until the late nineteenth century but have been decreased due to rodent control and sanitation measures. There are currently two natural populations of R. rattus left, one in the Lundy Islands and one in the Shiant Islands of Great Britain. Although rats pose a threat to native seabird species and their eggs, seabird populations have remained stable and large, and therefore control options for R. rattus have not been used.[27]